linux-kernel - Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE

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Message-ID: <4ef8a2aa44db971340b0bcc4f73d639455dd4282.camel@kernel.crashing.org>
Date:   Mon, 27 Aug 2018 18:09:50 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        Jann Horn <jannh@...gle.com>,
        Adin Scannell <ascannell@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE

On Mon, 2018-08-27 at 18:04 +1000, Nicholas Piggin wrote:
> > Yes.. I see that. tlb_remove_check_page_size_change() really is a rather
> > ugly thing, it can cause loads of TLB flushes. Do you really _have_ to
> > do that? The way ARM and x86 work is that using INVLPG in a 4K stride is
> > still correct for huge pages, inefficient maybe, but so is flushing
> > every other page because 'sparse' transparant-huge-pages.
> 
> It could do that. It requires a tlbie that matches the page size,
> so it means 3 sizes. I think possibly even that would be better
> than current code, but we could do better if we had a few specific
> fields in there.

More tlbies ? With the cost of the broadasts on the fabric ? I don't
think so.. or I'm not understanding your point...

Sadly our architecture requires a precise match between the page size
specified in the tlbie instruction and the entry in the TLB or it won't
be flushed.

Ben.